I have been studying the laws of kashrut as formulated by the Rambam in Hilkhot Ma’akhalot Assurot (Laws of Forbidden Foods) lately, and more in-depth than I ever have previously. Along the way, I have examined several elements of kashruth – particularly with regard to the status of kelim – that have bothered me for a while as either being unreasonable or illogical – i.e. out of step with visible and perceptible reality. One such subject is the kashrut of knives.
I have heard from various rabbis and mashgihim that knives which have been used to cut meat at any time, if they are used to cut an onion (a possible example of a davar harif – “sharp thing”) – EVEN IF THEY ARE CLEAN AND ARE COLD – the cut onion thereby “absorbs” the residue/taste of meat that are supposedly absorbed into the blade of the knife. As a result, according to those who maintain that this actually happens, the cut onion may thus not be eaten with dairy because it has now become “meaty.” This idea implies that there are invisible meat substances that we cannot perceive but are still there, and which present a problem halakhically. But as I reflected on this, it appears to fly in the face of other areas of kashrut where we rely on a principle of shalta be-`eina (“what is visible to the naked eye”). For instance, when it comes to insects on food, those which cannot be seen by the naked eye are halakhically insignificant, otherwise any microscope would no doubt reveal microscopic mites that feature on many things we are in daily contact with – even our own bodies. And if such a thing can be said about a bug – which is potentially forbidden de-oraitha – how can it not be said about supposedly invisible meat residue which is only forbidden de-rabbanan? I have suspected for a long time that there must be something lost in translation or a misunderstanding of Hazal in regard to knives.
My suspicions were delightfully confirmed upon reading the pirush of Rav Yosef Qafih (“Mori Yusef”) z”l on the Mishneh Torah. The following is my translation of a comment on Hilkhot Ma’akhalot Assurot 9:24. In it, Rav Qafih explains these laws from a sensible and reality-based perspective, one which brings the kashrut of knives and food cut by them back down to earth. Please note that all bolding, italics, bracketing, and underlining is for emphasis and is not in the original passage(s).
סכין שחתך בה בשר צלי וחזר וחתך בה צנון וכיוצא בו מדברים חריפין אסור לאוכלן בכותח אבל אם חתך בה קישות או אבטיח גורד מקום החתך ואוכל השאר בחלב
“A knife that was used to cut roasted meat, and it was then used again to cut ssenon [radish] – or things like it from the category of ‘sharp’ things – it is forbidden to eat them [i.e. the ssenon and similar things] with kutah [a thick dairy dip]. But if he used it afterward to cut kishut or avatiyah [types of melon], he scrapes the place where it was cut and he may then eat the rest with dairy.”
 In Hullin 111b, Hizkiyah says in the name of Abbaye, “Hilkhata [the final ruling is]… tzenon that one cut with a knife with which he cut meat – it is forbidden to eat it with kutah, and these things were said about tzenon since it, on account of its sharpness, will absorb, but with kishut he scrapes the place where it was cut and eats it [with kutah].” Behold, in the Gemara a knife used to cut roasted meat is not specifically mentioned, but only meat generally, and so it appears that Rabbenu [Rambam] is of the opinion that duhka (“pressing”) and harifuta (“sharpness”) never cause kelim to either give taste or absorb. And therefore, he adds “roasted meat” because by cutting roasted meat it will have lots of visible fat on it, and even though the following is not a formal proof of the matter, there is nevertheless an indication of this fact in what is brought in Beyssah 16a: “Rabbi Yisshaq says… the fat which is congealed on the surface of a knife, we scrape it off and rely on it for an eruv tavshilin.” And Rabbenu writes in Hilkhot Shevitat Yom Tov 6,4: “Even the fat that is congealed on the surface of the kind of knife that we use to cut roasted meat, it may be scraped together and if it contains the volume of an olive etc.” We see that according to Rabbenu a knife that we use to cut roasted meat usually has on it a thick layer of visibly congealed fat, such as that we can scrape from it an olive’s bulk. And therefore what is forbidden regarding ssenon is that, due to it’s sharpness, it will absorb the fat that is visible. This is not the case for something that is not sharp, since it merely wipes the fat off from the surface of the knife when it is cut and thus it is only necessary to scrape the fat from the place where it was cut. And in the printed edition, the redactors did not understand this emphasis of Rabbenu, namely that this halakhah deals specifically with a knife used to cut roasted meat, so they added the phrase “but if it was used to cut meat and then used again to cut kishut,” but this is not in the original handwritten manuscripts.
The Kesef Mishneh [written by Rav Yosef Qaro] writes here:
“In the chapter of Hullin entitled ‘Kol HaBasar‘ [page 112]: ‘Hizkiyah says in the name of Abbaye… ssenon that one cut with a knife with which he cut meat – it is forbidden to eat it with kutah, and these things were said about ssenon since it, on account of its sharpness, will absorb, but with kishut he scrapes the place where it was cut and eats it [with kutah]. Turnip stalks are permitted, but those of beets are forbidden. However, if he began by cutting a turnip and then proceeded to cut a beet stalk, then the beet stalks are permitted [to be eaten with kutah].’ And Rashi explains this passage as follows: ‘Even though it has been established for us that na”t bar na”t (i.e. transferred taste that is second-hand) is permitted, it is a different case to consider a knife that sometimes has congealed fat on it but is not recognizable as such. And when it cuts the tzenon, the na”t will come from the actual substance [i.e. it will be firsthand – na”t – and not second-hand – na”t bar na”t]. And further, due to its sharpness, it will absorb more that does boiled fish, and through the pressure exerted on the knife [duhka de-sakina], the knife will give taste and the tzenon will absorb it.’ And the Rashba writes: ‘The Rif did not write in the Halakhot regarding either turnip stalks or beet stalks, and neither did he include the statement that if one cut turnip stalks before he cut beet stalks, then the beet stalks [although sharp] are permitted [to be eaten with kutah]. Also the Rambam deleted it, and I have no idea why they both did so.’ And I maintain that Rabbenu, even though he did not write them explicitly, nevertheless they are included in his words in accordance with his explanation of the Gemara, for behold he wrote from ‘A knife that was used to cut roasted meat, and it was then used again…’ until ‘…he may then eat the rest with dairy.’ It appears that he is explaining that tzenon is lav davka [not specific, i.e. it could also something else similar to tzenon] since this is the rule for all sharp things, and it appears that he is also explaining that turnips are lav davka as well since this is the rule if one were to first cut bread or some other vegetable or fruit – because the prohibition to cut ssenon with such a knife is for no other reason than the congealed fat on its surface which strongly adheres to something sharp. So when that knife is first used to cut bread or any other thing, behold the knife is wiped clean thereby and no fat remains on its surface – so it is therefore permitted thereafter to cut tzenon with that knife. And now everything the Rabbenu states here is made clear, for from what he writes that ‘a knife that was used to cut roasted meat, and it was then used again to cut tzenon’ it is implied that what he means is: when a knife was used to cut the meat and then it was to cut ssenon without cutting anything else in between them and because of this lack of interruption it is forbidden. And since if he cuts something else between them it is thus permitted [!!!], there was no need for him to include the statement that ‘if he began by cutting a turnip and then proceeded to cut a beet stalk, then the beet stalks are permitted.’
And in regard to his statement that the knife was used to cut “roasted meat” being also lav davka since it is the same rule for meat that is either being cooked or boiled (however, there is room to say that he does specifically mean roasting meat like we said above because the fat of cooked meat is dissolved into the water and it does not congeal in a thick layer on the surface of the knife). And he did not employ the phraseology of “meat” by itself in order to teach us that the cutting of boiled meat is what we are discussing here, but with regard to cold meat the fat does not congeal on the surface of the knife to the point that it would be forbidden to cut tzenon with it [!!!]. And it is not necessary to write about beets since they are included in what he writes “or things like it from the category of ‘sharp’ things.” And for this reason it is also not necessary to write “turnip stalks are permitted” since turnips are not in “the category of ‘sharp’ things.” And in this way it is possible to reconcile the summation of the Rif and it appears to me that it would be difficult to do so if their text of the Gemara was like ours, but it is already possible – even without this issue – that these words [i.e. regarding turnips and beet stalks] were not written in their versions of the Gemara [and therefore they did not write them down]…
This is nothing short of AMAZING! The implications of this beautiful and cogent explanation of the halakhah are apparent to anyone who is familiar with the traditional understanding of these laws. Basically, Rav Qafih is saying here that only visible, actual fat adhering to the blade is of halakhic concern – no invisible, lurking fat that is somehow [invisibly] “possessing” our kitchen cutlery to worry about!
Please let me know your thoughts and comments on this. This understanding completely revolutionized my understanding of kashruth in general and that of knives specifically.